The present invention relates generally to the preparation of carbon materials for use as electrodes in rechargeable batteries and more particularly to methods of treating polymeric precursor powders and fibers and producing carbon materials for use as anode materials in rechargeable lithium batteries having improved performance.
A majority of the research aimed at development of rechargeable batteries that exhibit improved performance characteristics, such as increased cycle life and energy and power densities, has focused on the development of lithium rechargeable batteries because they provide significant advantages in performance characteristics when compared to other battery systems. Of particular interest, has been the development of lithium anodes for secondary battery applications.
Rechargeable lithium battery cells that utilize lithium metal as an anode material have not gained widespread use due to limitations in cell performance resulting from extensive dendrite formation leading to cell shorting and inefficient electrochemical deposition of lithium on charging, coupled with safety problems inherent in the use of lithium metal, which is highly reactive. As disclosed by Murakami et al. in U.S. Pat. No. 4,749,514, many of these problems can be overcome by incorporating lithium into a graphitic carbon structure. This process, known as intercalation, involves insertion of lithium metal atoms along the c-axis of graphite to form a charge transfer compound, wherein the lithium atom appears to donate an electron to the graphite/carbon host binding the lithium to the graphite/carbon host by electrostatic attraction. By incorporating lithium into a graphite/carbon host in this fashion the chemical reactivity of the lithium is reduced, overcoming problems associated with the use of metallic lithium.
Carbon in various physical forms (foams, powders, fibers) and states of aggregation (films, monolithic pieces, pressed powders/fibers) has been used for many years as an electrode material in batteries. The synthesis of carbonaceous materials for lithium intercalation anodes has been extensively described. These syntheses generally involve the controlled pyrolysis of an organic precursor material such as benzene (Mohri et al., U.S. Pat. No. 4,863,814; Yoshimoto et al., U.S. Pat. No. 4,863,818 and Yoshimoto et al., U.S. Pat. No. 4,968,527), selected furan resins (Nishi et al., U.S. Pat. No. 4,959,281), thin films of poly(phenylene oxadiazole) (Murakami et al., U.S. Pat. No. 4,749,514), various carbonizable organic compounds such as condensed polycyclic hydrocarbons and polycyclic hetrocyclic compounds, novalak resins and polyphenylene and poly(substituted) phenylenes (Miyabayashi et al., U.S. Pat. No. 4,725,422; Hirasuka et al., U.S. Pat. No. 4,702,977).
By way of example, Arnold et al., U.S. Pat. No. 4,832,881 and Simandl et al., U. S. Pat. No. 5,208,003, describe carbon materials in the form of foams, aerogels and microcellular carbons which are useful as anode materials for high energy density batteries. While these carbon materials represent an improvement over conventional carbon powder for use as anodes, they have several disadvantages. Methods used to prepare these carbon materials require elaborate processing steps to prepare their precursor materials; among other things, solvents used to prepare the precursor materials must be completely removed from the precursor materials prior to the carbonization step. In order not to disrupt the microstructure of the precursor material the solvent removal step must be done under carefully controlled conditions using, for example, freeze drying or supercritical extraction. Furthermore, the solvents must either be disposed of or purified if they are to be reused. In addition, before the carbonized product produced by these processes can be used, additional fabrication steps, such as machining, must be employed.
For the reasons set forth above, there has been a particular interest in developing carbon materials that will reversibly intercalate and deintercalate lithium. However, many of the carbon-based systems initially developed were not able to provide high cycle life due to limitations of the graphite/carbon electrode material, e.g., exfoliation during cycling and/or reaction with the solvent. Further work has led to development of carbon materials that are able to cycle well, and battery cells utilizing these materials are commercially available. However, these carbons are typically monolithic materials, having high surface areas, which limit their usefulness, particularly for secondary battery applications. Furthermore, they are difficult and expensive to manufacture.
In addition to new carbon electrode materials that are more compatible with lithium, there have been numerous efforts to improve the intercalation efficiency of carbon materials useful for lithium intercalation electrodes. One solution is described in Yoshino et al, in U.S. Pat. No. 4,668,595, wherein doping of a wide variety of carbons formed from carbon powders, carbon blacks and carbonized polymeric fibers is disclosed. Azuma et al., U.S. Pat. No. 5,093,216 disclose incorporation of phosphorous into carbonized materials to improve intercalation efficiency and Mayer et al., in U.S. Pat. No. 5,358,802, disclose doping carbon foams with dopants such as phosphorous, boron, arsenic and antimony to improve intercalation efficiency. However, these carbon materials showed poor cycle life and one problem that still remains to be overcome is the irreversible loss of lithium that takes place during initial cycling of these carbon material as an electrode in a battery environment. The irreversible losses of lithium from the carbon electrode materials can result in the loss of 30 to 60% of the initial battery capacity.
What is required is a carbon material that can be fabricated into electrodes for lithium secondary batteries that exhibits high intercalation efficiencies for lithium, low irreversible loss of lithium, long cycle life, is capable of sustaining a high rate of discharge and is cheap and easy to manufacture.
Responsive to these needs, novel processing methods have been developed for producing carbon materials for use as electrodes in rechargeable batteries. Polymeric precursor materials processed in accordance with the present invention can yield carbon materials for use as electrodes that exhibit high intercalation efficiencies and in which the irreversible loss of lithium can be reduced to a few percent of the initial capacity. Furthermore, the lengthy and involved extraction procedures for removing solvents can be eliminated thereby reducing the cost of producing the carbon material. In addition, carbon materials having higher densities can be obtained, thereby making it possible to achieve high energy density batteries. In particular, the present invention can improve the performance of alkali metal secondary batteries by the use of anodes prepared from treated polymeric precursor materials. Additionally, lithium intercalation electrodes prepared from polymeric precursor materials processed in accordance with the present invention exhibit minimal dendritic deposition, have long cycle life and are capable of sustaining the high rate of discharge required for high energy density secondary batteries. Electrodes prepared from such treated polymeric precursor materials can also retain a large fraction of their initial capacity.